Water become challenging source in my native (Tamilnadu, India). While I build new home, it makes me think about saving the rainwater.

Ground water(borewell) is the only water source for us. This also having chance to dry on summer. So I like to use the rain water in efficient way.

Is it good to mix the ground and rain water? or should have two separate water storing tanks(underground). Which one is cost effective?

I was thinking to use the two tanks for two different purpose.

Rain water (After basic filtering)

  • Toilet
  • Washing
  • Terrace Garden
  • Kitchen Sink

Ground water

  • Drinking (Purified)
  • Both
  • Cooking

Needless to say, I also plan to save the good water from kitchen sink(double sink method) and purifier waste water in barrel for ground garden plants.

So please advice me?

  • Are you storing potable groundwater for a length of time?
    – isherwood
    Commented Apr 12, 2016 at 20:07
  • Tank usually comes for 10 days. Main purpose of the tank. When bore well stop working in summer, we usually buy water for money in tankers. So we should have the storage tank for that!
    – Gowri
    Commented Apr 12, 2016 at 20:11

1 Answer 1


Rain water has (practically) no electrolytes. We've all heard that rainwater is acidic, but realistically, that is only relevant on a larger, ecological scale (tons of slightly acidic water will cause erosion). More importantly, rainwater will contain nitrates, oils, and other organic chemicals, and rainwater will contain lots of microorganisms, which will be difficult to filter out with an ordinary (ground-water) filtration system. Common ground-water filters are between 5 and 20 microns. Filtering water is extremely helpful for removing bacteria, but the water is still not sterile. Holding the rainwater may allow growth of bacteria, if the rainwater is "nutrient" rich.

The earth filters out nutrients and bacteria. It's not a perfect system, but groundwater is drinkable (usually after basic filtering). Rainwater should not be poured directly into ground water. Keeping the systems isolated would keep contamination minimalized. In terms of cost, try to think of this as rainwater will pollute the groundwater and could possibly ruin the well. Rainwater treatment can make it potable/drinkable, but I would reserve that option for the worst case scenario unless you have a good treatment system.

At a minimum, rainwater treatment (to make it potable) should include coarse and fine (5 micron) filtration, chlorine and UV sterilization, and carbon filtration.

I would do this:

Rain water (in order of priority)

  • Toilet


    Outdoor hose

Ground water (in order of priority)

  • Drinking (Purified)


    Kitchen Sink

    Washing Dishes



The list is prioritized such that if groundwater is low, then laundry could be done with rainwater. If things get bad, you could use rainwater for bathing. I would try to avoid washing dishes and cooking with rainwater. I would drink rainwater if my life depended on it.

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